Basic story
At its most basic, the story tells of a young scholar who falls in love with a beautiful woman, unaware that she is a white snake who has taken on human form. A monk intervenes in order to save the scholar's soul and casts the white snake into a deep well at the Leifeng Pagoda.
Over the centuries the story has evolved from horror story to romance with the scholar and the white snake-woman genuinely in love with one another, but such a relationship is forbidden by the laws of Heaven. There have also been variations on the telling of the story: like the scholar adopting the white snake as a pet while still a schoolboy; or himself being banished from Heaven and becoming a mere human on Earth.
An added character is a green snake (or in some cases, a carp) who has also been turned into a woman and serves as the white snake-woman's soul sister and confidante.
Storyline
The story is set in the Southern Song Dynasty.
A female white snake demon, Bai SuZhen, dreams of transforming into a goddess by doing good deeds, so she takes on human form and goes to the human realm. There she meets a green snake demon, Xiao Qing, who causes disasters in the area she lived. Bai SuZhen holds her captive at the bottom of a lake, though she promises the green snake that she'll come back in three hundred years to free her.
After three hundred years she keeps her promise and frees her. They bond as sisters. They meet a sorcerer called FaHai who believes that every demon should be destroyed. But Bai was too powerful and he can't eliminate her immediately so he vows that he will if he sees them again.
Fearing that they would meet more human sorcerers, Bai and Qing retreat into the world between demon and human world called Ban Bu Duo where they try to do good things by bringing rain to places that hasn't had any water for three years. But Qing was careless and almost flooded the whole town. because of this, Bai sadly loses her chance of becoming a goddess. However Guan Yin informs her that she may have yet another opportunity.
In the meantime Bai and Qing accidentally brought a scholar Xu Xian and his friend to the demon world. Bai protected them from demons and in the process fell in love with Xu Xian. After the battle with the Leader of the underworld, Xu Xian confesses his feelings for Bai, claiming that it was love at first sight. But in order for a human to go back to the human world they have to become unconscious and in the process also forget their time in the demon world. Xu Xian knows and avoids being knocked out. However, FaHai somehow found a way into the demon world and seeing Xu Xian, tricks him into being knocked out.
Now Xu Xian is back into the human world and has forgotten everything. Since he and his friend went through the portal separately they land in separate places. There Xu Xian meets many new people.
Soon after Bai takes the final step to becoming a goddess which is to collect human tears. Bai sees Xu Xian with another girl and assumes that they are a couple. The former green snake, Xiao Qing, realises that when Xu Xian and Bai meet, Xu Xian will again fall in love with Bai and so arranges a meeting. They then got married, opened a medicine shop and lived happily.
But as demons and humans are not supposed to join, the town was struck by a plague and it was soon on the verge of total destruction. Bai, Qing and FaHai finally agreed to a truce and obtained the magical herb needed to help the population.
Later Bai gets pregnant, but Fahai continues to try to eliminate her and Qing.
On the fifth day of the fifth month, the Dragon Boat Festival is held. On that day demons revert to their true selves. Bai thus decides to take Qing and Xu Xian back to Ban Bu Duo, but Xu Xian falls for FaHai's tricks yet again and Bai shows her true self, scaring Xian literally to death. Bai retrieves the herbal medicine which brings Xian back to life.
But after giving birth to a son Bai can't control herself anymore and is forced to tell her husband the truth about her origins. Xian kindly accepts her, but Fahai then attacks the weakened Bai and holds her to eternal captivity in the Leifeng Pagoda.
